1. Field of the Invention
The present invention relates to a cooling device, a cooling method employing the cooling device, and an image forming apparatus, such as a copier, a printer, a facsimile machine, a plotter, or a multifunction machine capable of at least two of these functions, incorporating the cooling device.
2. Description of the Background Art
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices including at least two of those functions, etc., include an optical writing device (exposure device) to direct writing light onto an image carrier so as to form an electrostatic latent image thereon, a development device to develop the latent image with developer, a transfer unit to transfer the developed image (toner image) onto a sheet of recording media, and a fixing device to fix the toner image on the sheet.
It is known that, in typical image forming apparatuses, devices such as the optical writing device, the fixing device, the development device, and a drive motor that drives the image carrier generate heat.
In recent years, as electrophotographic image forming apparatuses, there is market demand for multicolor image forming apparatuses, such as multicolor multifunction machines and multicolor printers. Some multicolor image forming apparatuses are so-called single-drum type image forming apparatuses in which multiple development devices for corresponding colors are provided around a single photoreceptor. In this single-drum type, toner images are formed on the photoreceptor by adhering the toner in the development devices, and the toner images on the photoconductor are transferred onto a sheet as a color image. Other multicolor image forming apparatuses are so-called tandem-drum type image forming apparatus in which multiple development devices for corresponding colors are provided around multiple respective photoreceptors. In this tandem-drum type, a single toner image is formed on each of the photoreceptor, and the single-color toner images on the respective photoconductors are subsequently transferred onto the sheet as a color image.
Comparing single-drum type and the tandem-drum type, in the single-drum type image forming apparatus, the image forming apparatus includes the single photoreceptor, which can be made more compact, thereby reducing cost. However, a full color (multiple color) image is formed by forming images several times (four or five times) using the single photoreceptor, which hinders an increase in image formation speed (printing speed). By contrast, in the tandem-drum type image forming apparatus, although the image forming apparatus is bulky and is relatively costly, it facilitates faster printing speeds. Therefore, at present, to improve productivity, it is desired to increase full-color printing speed to levels like those of monochrome printing, and for this reason tandem-drum type image forming apparatuses have been drawing attention.
Some tandem-drum type multicolor image forming apparatuses are direct-transfer types (see FIG. 1), in which toner images on photoreceptors 211 in photoreceptor units 210 are subsequently transferred onto a sheet P that is conveyed by a sheet conveyance belt 250 and respective transfer members 251. Others are indirect-transfer types (see FIG. 2), in which images on the photoreceptors 211 in the photoreceptor units 210 are subsequently transferred onto an intermediate transfer belt 260 by primary transfer members 261, after which the images on the intermediate transfer belt 260 are transferred onto a sheet P all at once by a secondary transfer device 270, which may be either a roller or a belt. In some indirect-transfer types, the intermediate transfer belt 260 may be disposed above the respective photoreceptor units 210 as illustrated in FIG. 3.
In the indirect-transfer tandem-drum-type image forming apparatus shown in FIG. 2, to make the image forming apparatus compact, in addition to packing components densely in the image forming apparatus, a fixing device 280 is disposed beneath the photoreceptor units 210 and adjacent to the respective photoreceptor unit 210. However, the fixing device 280 generates heat that can affect the temperatures of the photoreceptor units 210.
At present, due to increasing demand for increase in the printing speed, more compact image forming apparatus, and higher image quality, the temperature increase in the respective photoreceptor unit (image forming unit) becomes an issue not only in the indirect-transfer-drum type image forming apparatuses but also in all image forming apparatuses. In addition, packing components densely in the electrophotographic image forming apparatus increases the amount of heat generated. Accordingly, failure, for example the toner used to develop images might congeal, may occur in the respective hot photoreceptor units.
In order to solve the above-described problem, such image forming apparatuses typically include forced-air-cooling devices in which air flows through a small area formed by a heat conductor provided in the development device and forcibly cools the development device. However, toner with a lower melting point has come to be widely used in the image forming apparatus to improve image quality and enhance performance. Therefore, it becomes difficult to secure sufficient cooling ability by air cooling.
In view of the foregoing, liquid-cooling devices have been proposed for cooling the devices in the image forming apparatus. In general, the cooling efficiency of liquid-cooling devices is higher than that of typical air-cooling devices. However, cooling is performed even when the ambient temperature is low and cooling is not necessary. In addition, since the image forming unit includes a cleaning blade in a cleaning device for clean a photoreceptor, a cleaning failure may occur when the cleaning blade is cooled too much.
Other known image forming apparatuses uses a liquid-cooling device that includes multiple heat receiving portions corresponding to image forming units (hot portions), multiple heat releasers (cooling members) corresponding to at least one image forming unit, a cooling tube through which coolant is circulated, a conveyance device to convey the coolant, and a controller. However, even with such a configuration, the problem of cleaning failure caused by excessive cooling remains unresolved.